Azide\nPhotochemistry in Acrylic Copolymers for Ultraviolet\nCross-Linkable Pressure-Sensitive Adhesives: Optimization, Debonding-on-Demand,\nand Chemical Modification

Abstract

Pressure-sensitive\nadhesives (PSAs) are usually made from viscoelastic,\nhigh-molecular-weight copolymers, which are fine-tuned by adjusting\nthe comonomer ratios, molecular weights, and cross-link densities\nto optimize the adhesion properties for the desired end-use. To create\na lightly cross-linked network, an ultraviolet (UV) photoinitiator\ncan be incorporated. Here, we present the first use of perfluorophenylazide\nchemistry to control precisely a polyacrylate network for application\nas a PSA. Upon UV irradiation, the highly reactive nitrene from the\nazide moiety reacts with nearby molecules through a C–H insertion\nreaction, resulting in cross-linking via covalent bonding. This approach\noffers three benefits: (1) a means to optimize adhesive properties\nwithout the addition of an external photoinitiator; (2) the ability\nto switch off the tack adhesion on demand via a high cross-linking\ndensity; and (3) a platform for additional chemical modification.\nA series of poly­(<i>n</i>-butyl acrylate-<i>co</i>-2,3,4,5,6-pentafluorobenzyl acrylate) or poly­(PFBA-<i>co</i>-BA) copolymers were synthesized and modified post-polymerization\ninto the photo-reactive poly­(<i>n</i>-butyl acrylate-<i>co</i>-4-azido-2,3,5,6-tetrafluorobenzyl acrylate) [azide-modified\npoly­(PFBA-<i>co</i>-BA)] with various molar contents. When\ncast into films, the azide-modified copolymers with a high azide content\nachieved a very high shear resistance after UV irradiation, whereas\nthe tack and peel adhesion decreased strongly with the increase in\nazide content, indicating that excessive cross-linking occurred. These\nmaterials are thus photo-switchable. However, in the low range of\nazide content, an optimum probe tack adhesion energy was obtained\nin films with a 0.3 mol % azide content, where a long stress plateau\n(indicating good fibrillation) with a high plateau stress was observed.\nAn optimum peel adhesion strength was achieved with 0.5 mol % azide.\nThus, the adhesion was finely controlled by the degree of cross-linking\nof the PSA, as determined by the azide content of the copolymer chain.\nFinally, as a demonstration of the versatility and advantages of the\nmaterial platform, we show an azide–aldehyde–amine multicomponent\nmodification of the azide copolymer to make a dye-functionalized film\nthat retains its adhesive properties. This first demonstration of\nusing azide functionality has enormous potential for functional PSA\ndesign.